Various cellular radio systems or mobile telephone systems are in use and presently being designed in which the geographical coverage area of the system is divided into smaller separate radio areas, i.e. cells, in such a way that when the radiotelephone or mobile telephone is located in a cell, it communicates with the fixed network through a fixed radio station incorporated in the cell. The mobile telephones belonging to the system can roam freely within the system area from one cell to another. One such system is the digital mobile telephone system GSM (Global System for Mobiles).
Cordless telephone (CT) systems, such as the DECT system, have recently come out on the market. The DECT system comprises a base station, which is normally based on access to an existing fixed telephone network. There are three main types of base stations: a residential base station, a base station connected to a private branch exchange in an office, and a telepoint base station.
Cordless telephone systems can also be connected to mobile telephone networks, for instance to the GSM system. The private branch exchange (PBX) of a cordless telephone system can be connected to the GSM system for example by employing DSS 1, Q.930-Q.940, 1989, Digital Subscriber Signalling System No. 1, Network Layer, User-Network Management, CCITT signalling supplemented with mobility management messages. The PBX of a cordless telephone system is connected to the switching centre of the GSM system so that the subscribers in the PBX can roam within the service area of the GSM system, and the subscribers in the PBX can make use of the mobility management procedures of the GSM system and roam within the service area of DECT systems associated with the GSM system. In such a case, mobile subscribers are attached to the PBX in accordance with the DECT radio path protocol ETS 300175, Radio Equipment and Systems (RES), Digital European Cordless Telecommunications (DECT), Common Interface, for instance. It has also been possible to connect fixed subscribers to the PBX, for example in accordance with the ISDN signalling protocol. In that event, calls originating from the GSM system or from a GSM-connected telephone network can also be routed to cordless, for instance DECT, subscribers located within the service area of the PBX.
In the prior art solution, wherein a subscriber located within the service area of a private branch exchange wishes to establish a connection, for example make a call or set up a telecommunications connection to another subscriber, register interrogation procedures in accordance with the GSM recommendations are performed, irrespective of the location of the called subscriber. FIG. 1 shows call establishment procedures in accordance with the prior art. In the figure, a subscriber CPP (Cordless Portable Part) in a private branch exchange PBX sends 101 a connection establishment request CC.sub.-- SETUP, by way of which the subscriber requests the PBX to establish a connection, to another subscriber. Upon receipt of the connection establishment request CC.sub.-- SETUP, the PBX sends 102 a service request DM.sub.-- SERVICE.sub.-- REQ to the mobile services switching centre MSC. Having received the service request, the MSC sends 103 an access request pertaining to mobility management, PROCESS.sub.-- ACC.sub.-- REQ, to the visitor location register VLR. The next step is the assignment of a temporary mobile subscriber identity TMSI.sub.-- ASSIGNMENT 104. This is followed by authentication 105 of the subscriber. Authentication is a process wherewith the subscriber is identified as a legal user of the system. Subsequent to the authentication, ciphering 106 of the messages employed is activated. Further, the visitor location register VLR responds to the access request PROCESS.sub.-- ACC.sub.-- REQ by sending 107 an acknowledgement ACCESS.sub.-- REQ.sub.-- ACC. The PBX subsequently or simultaneously acknowledges the received CC.sub.-- SETUP message by sending 108 an acknowledgement message CC.sub.-- SETUP.sub.-- ACK to the subscriber CPP. Having received an indication of a successful connection establishment request, the subscriber sends 109 the information needed for completing the connection, for example the subscriber number of the B-subscriber, to the private branch exchange PBX in a CC.sub.-- INFO message. The PBX further transmits 110 this information and the connection establishment request as a SETUP message to the mobile services switching centre of the subscriber. The mobile services switching centre sends 112 a subscriber data check interrogation S.sub.-- INFO.sub.-- F.sub.-- O/G.sub.-- CALL to the visitor location register VLR, which checks the subscriber data in its subscriber database. When the visitor location register has checked the subscriber data, it sends 113 a COMPLETE.sub.-- CALL message as an indication of a successful call to the mobile services switching centre MSC, which transmits 114 a CALL.sub.-- PROC message to the private branch exchange PBX of the subscriber. The PBX transmits an indication of a successful call to the subscriber CPP by sending 115 a CC.sub.-- CALL.sub.-- PROC message. As a next step, the MSC sends 116 an indication of the receiving subscriber, the inbound call and the roaming number in an initial address message IAM (specified in the common channel signalling standard Q.721-Q.766, 1989, Specifications of Signalling System No. 7, CCITT, Vol. VI--Fascicle VI.8, p. 251) to the mobile services switching centre MSC of the location area of the receiving subscriber, which may also be a gateway mobile services switching centre GMSC. It is also to be noted that the receiving subscriber may be located within the service area of the mobile services switching centre MSC of the location area of the subscriber CPP requesting the connection, in which case the IAM message is sent as an internal routine of the mobile services switching centre. If the IAM message has been sent to a gateway mobile services switching centre GMSC, the GMSC sends 117 an ACM message (Address Complete Message) in response to the IAM message, thus indicating that all digits required for routing the call have been received. The mobile services switching centre MSC then sends 118 an ALERTING message to the private branch exchange PBX, which responds to the CC.sub.-- SETUP message by sending 119 a CC.sub.-- ALERTING message to the private branch exchange PBX. When the receiving subscriber, i.e. B-subscriber, answers the call, the gateway mobile services switching centre GMSC sends 120 an ANM message (Answer Message). Thereafter the mobile services switching centre sends 121 a CONNECT message to the private branch exchange PBX, which transmits 122 said message to the subscriber CPP as a CC.sub.-- CONNECT message. The PBX acknowledges successful sending of the CONNECT message by sending 123 an acknowledgement message, CONNECT.sub.-- ACK, to the mobile services switching centre.
It is apparent from the foregoing that establishment of a connection to a subscriber located within the service area of a private branch exchange connected to a cellular radio network is a complicated and cumbersome process. It should be noted that the prior art call establishment procedure as set forth above is to be performed every time a subscriber located within the service area of the private branch exchange wishes to establish a call or another telecommunications connection. The mobility management functions to be performed in the GSM system which form part of the operations for connection establishment, i.e. the subscriber authentication 105, ciphering 106 and TMSI assignment 104 and the PROC.sub.-- ACCESS.sub.-- REQ 103 and COMPLETE.sub.-- CALL 113 messages to be sent, as described in the GSM 09.02 recommendation, considerably load the signalling capacity of the mobile telephone network. Heavy loading of the signalling capacity may lead to an overload and system failure, and naturally gives rise to unnecessary traffic in the network. It is further to be noted that the above connection establishment procedures and sending of the messages mentioned above load the network elements that transmit such messages. The processor capacity of the network elements is unnecessarily occupied and may be overloaded. Furthermore, the network elements receiving the messages must process them and perform the desired functions which again load further network elements that receive messages.